Time division multiplexing is a type of digital switching in which a line is time shared among multiple users, and in which multiplexed time slots are allocated for establishing communication sessions.
The art of digital time division switching also includes systems known as Time Slot Interchangers (TSI), in which digital message samples from a given input time slot are switched to an alternate output time slot. TSI is now used to establish conference connections.
U.S. Pat. No. 4,119,807; entitled "Digital Time Division Multiplex Switching System" and issued Oct. 10, 1978; is an example of a time shared digital switch arranged to establish conference connections without restriction as to the number of conferees that may participate in a conference session. This patent discloses an arrangement for sequentially summing digital message samples from 128 lines connected respective to 128 time slots during a first time frame and outputting the summed samples to each line of the connection, respective to its assigned time slot, during a second time frame.
A digital switching arrangement of this type uses two summation memories, which are alternately loaded and unloaded respective to a time frame. In addition, third and fourth memories are alternately loaded in step with the summation memories, and are used for the storage of digital message samples respective to assigned time slots. Each summation memory is cleared at the beginning of its respective storage cycle or time frame in preparation for the storage of a new series of summed message samples.
Currently, broadcast polling multipoint connections consisting of a master station and one or more remote stations are typically established using analog conference bridges. The master station or host broadcasts polling information to the secondary legs of the multipoint or conference connection and transmissions from the secondary legs are received only by the host station.
In a typical operation, requiring 4-wire channels, the host station polls each secondary leg by transmitting a unique address recognized by the polled station. If the polled station has no business to transact with the host it returns a negative or no response and the host station polls another remote station. If the polled station has business to transact with the host, the polled station returns a positive response and the transaction is completed before the host polls another station. All data transactions between remote stations are made through the host station, since typically, the multipoint conference network is designed to isolate remote stations from each other. This requirement is necessary to prevent data from a polled station or noise from an unpolled station from confusing other remote stations.
Presently, broadcast polling multipoint connections are typically established using 4-wire circuits having an analog bridge in the transmit and receive paths. In addition, each remote station is connected to the receive bridge via an amplifier circuit having forward but no reverse gain to isolate remote stations from each other.
The data conference connection described above is not only expensive to construct but is expensive to maintain. Analog circuits, especially amplifiers, require scheduled maintenance to adjust the gain of the amplifiers, which drift over a period of time, and to prevent cross-talk between the channels. Also, each conference network must be specially engineered to meet user requirements as to the number of conferees that will be connected to the network. A specially engineered conference network is an expensive undertaking.
Most prior digital conferencing systems, including U.S. Pat. No. 4,119,807, process digital voice samples and therefore have the capability to store and forward digital data. However, a significant shortcoming inherent in such systems is the inability to properly process or control data streams passing between a conference consisting of a host computer and secondary computers, such as a broadcast polling multipoint connection. In prior art systems data transmitted from a polled computer to the host computer is also forwarded to other computers in the conference network since data or noise samples from each station in the network are summed and stored during a read cycle and supplied to each station (time slot) during a supply cycle. Therefore, data not only passes between the polled computer and host computer but also passes to non-polled computers. Also, the last data stream from a priorly polled computer may be summed with a command from the host computer to a newly polled computer thereby corrupting the command. Since the command is embedded in a summed data stream it may be unrecognizable by the newly polled computer. Consequently, the newly polled station may disregard the command and remain idle.
In essence, prior art digital conferencing systems lack the ability to isolate secondary stations from each other and also lack the ability to integrate both voice and data connections.